Speaker
Description
We show that using renormalization-group summation to generate the QCD radiative corrections
to the $\pi-\gamma$ transition form factor, calculated within lightcone sum rules, leads the
strong coupling free of Landau singularities while preserving the QCD form-factor asymptotics.
This enables a reliable applicability of the LCSR method to momenta well below 1~GeV$^2$.
This way, one can use the new preliminary BESIII data with unprecedented accuracy below
1.5~GeV$^2$ to fine tune the prefactor of the twist-six contribution.
Using a combined fit to all available data below 3.1~GeV$^2$, we are able to determine all
nonperturbative scale parameters and a few Gegenbauer coefficients entering the calculation
of the form factor.
Employing these ingredients, we determine a pion distribution amplitude with conformal
coefficients $(b_2,b_4)$ that agree at the $1\sigma$ level with the data for
$Q^2 < 3.1$~GeV$^2$ and fulfill at the same time the lattice constraints on $b_2$
at N$^3$LO together with the constraints from QCD sum rules with nonlocal condensates.
